1. Field of the Invention
This invention relates to extrusion dies for vulcanizable rubbery materials. In one of its aspects, the invention relates to a multiple-cavity extrusion die for simultaneously coextruding a plurality of multiple-layer tubular members at equal rates. In another of its aspects, the invention relates to a multiple-cavity extrusion die for extruding multiple-layer tubes wherein at least some of the dies are adjustable for concentricity of the core and cover layers independently of each other. In still another of its aspects, the invention relates to a novel die structure for coextruding one or more multiple-layer tubes of vulcanizable rubbery material.
2. State of the Prior Art
Rubber tubing for many industrial applications requires that the material be resistant and impervious to certain organic solvents and gasoline. Many materials such as polymers of ethylene, propylene diene monimers and neoprene are suitable for such uses. However, these rubbery materials are expensive and not always available in large quantities. It has thus been popular to make rubber tubing in two adhesively secured concentric layers. The inner or core layer is thinner and made of a higher priced barrier material. The outer or cover layer is thicker and made of a less expensive, perhaps more durable, and more readily available material such as butyl rubber or GRS.
Heretofore such composite tubes have been made in a two-step process. First, the core layer was extruded to a desired thickness. The extruded core tube was then drawn through a cross-head die and the cover layer was extruded on top of the core tube. This process is rather expensive in that it requires a dual line of equipment and is considerably more time consuming than necessary. Insofar as presently advised, equipment for simultaneously extruding two concentric tubular layers of rubbery materials has not been heretofore successfully developed.
Devices for coextrusion of tubular shapes with concentric layers are described in the patent literature and are well known in the thermoplastic extrusion art. U.S. Pat. No. 1,637,207, issued July 26, 1927, to Whitehouse, for example, discloses an apparatus for coextruding tubes of rubbery material to form concentric tubes. As is thought necessary in the extrusion art, long land dies are used to extrude the core material and a relatively long tapering extrusion cavity is shown for extrusion of the cover layer.
Long land dies and long extrusion cavities are generally thought necessary for proper working of plastic materials prior to extrusion. Such structures are desirable for thermoplastic materials. However, the long and narrow extrusion cavities have proven to be undesirable for thermosetting materials, such as vulcanizable rubbers. These rubbery materials have a narrow range of extrusion parameters; they set under the influence of temperature and time. Thus, the higher the temperature, the shorter the time in which the materials set. The working of the rubbery materials in the elongated narrow extrusion cavity raises the temperature of the rubbery material significantly and makes temperature control of the rubbery material in the extrusion cavities difficult. Excessive heating of the rubbery material can cause setting of these materials in the dies. It has accordingly been found that the long tapered extrusion cavities, as, for example, disclosed in the Whitehouse patent, are unsuitable for coextrusion of most thermosetting rubbery materials because of the difficulty of controlling the temperature of the rubbery polymer and dwell time of the polymer in the dies.
U.S. Pat. No. 253,822 to Mignon, issued Feb. 14, 1882, discloses the coextrusion of materials to form a candle for electric lighting. The candle has a tubular carbon cover layer and a solid core or plastic material core layer. An extrusion device for manufacturing arc lamp electrodes is also disclosed in U.S. Pat. No. 830,201 to Blondel, issued Sept. 4, 1906. However, neither the Mignon nor the Blondel extrusion apparatus would be suitable for extruding the rubbery material in tubular form because of the long extrusion cavities.
An extrusion die for extruding reinforced plastic or rubber tubing is disclosed in U.S. Pat. to Schiesser, No. 3,697,209, issued Oct. 10, 1972. In the Schiesser die, a rubbery or plastic material is extruded in a first tubular shape on a mandrel and reinforcing material is thereafter fed through the die onto the tubing. Another layer of the same rubbery material is thereafter extruded onto the reinforced core layer. The mandrel is mounted on a pair of eccentric sleeves which are used to adjust the mandrel for concentricity when rotated. An annular disc forms the outer surface of the reinforced tubing and is mounted in a holder through a series of set screws which can be adjusted for centering the outer tubing with respect to the central mandrel. The Schiesser die is not adopted to extrude two different rubbery materials.
The extrusion of small composite tubing from a single die head would be significantly more economical if multiple tubes could be extruded simultaneously from the same die head. However, multiple die head extrusion processes frequently cause problems in that the tubing is not extruded at the same rates and the concentricity of the tubing varies depending on the relative position of the die opening with respect to the feed system.